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TECHNICAL LIBRARY

Simulation of Ion Irradiated Power Devices in ATLAS

About ten years of evolution was sufficient for ion irradiation
technology to become a widely used tool for local carrier lifetime
control in power devices. In 1994, the simulation of devices taking
into account both the real defect profile resulting from ion irradiation
and multi-level Shockley-Read-Hall model was published for the first
time [1]. ATLAS has allowed simulation of transient traps since
1995 [2]. The last version of ATLAS (4.0) brought the possibility
to account for arbitrary defect spatial distribution. So the development
of ion irradiated devices using device simulation is now possible
[3]. At present, any application of the simulator requires just
a knowledge of the spatial distribution of the defects resulting
from irradiation and electrical parameters of the related deep levels
[4]. The practical application of this will be presented below for
the case of power diode.

Background

Deep levels generated by ion irradiation affect the free-carrier
thermal generation-recombination and hence the excess carrier lifetime.
The thermal capture and emission of carriers through deep levels
located within the bandgap is described in ATLAS by analytical model
based on SRH statistics. In case of k independent single-charged
acceptor- or donor-like levels the thermal components of the recombination
rates Rn and Rp for electrons
and holes, are respectively [1-3]:

where Nti is the concentration of the
i-th deep level and Gn(p) and Kn(p)are the i-th level emission and capture rates for electrons
(holes). The electron occupancy fi of the i-th deep level is calculated
from the following balance equation

The charge of traps Dt=(pt-nt)
influences the right-hand side of the Poisson
equation

Application of this model requires a detail knowledge of deep
level parameters. The ATLAS command is

ATLAS also incorporates a model for transient trapping and
de-trapping of carriers. For dynamic equilibrium dfi/dt=0
(DC analysis) CPU time is saved because the recombination rate R
is unique and may be expressed explicitly by one equation in the
following way

Transition between the two states is automatic in ATLAS. There
is available a second SRH model (models srh) which uses the static
approximation even for transient analysis. In this case only a single
G-R centre is considered and the equation above leads to the fairly
well known formula

where k is Boltzmann constant, T
is the temperature, niis intrinsic
concentration, Eiintrinsic Fermi-level,
Et trap level, tn(p)0=1/(sn(p).vn(p).Nt) are electron and hole lifetimes,
resp. It is worth reminding that this equation is true only for
a single ideal G-R center (Rn=Rp)
or dynamic equilibrium, (e.g. ON- or OFF-state).

Defining Trap Parameters

The device under consideration is a p+pnn+ power diode (2.5kV/100A)
with length of 370µm between anode and cathode. The detailed
device data may be found in [5]. In order to present the simulation
capabilities of ATLAS the device under test was virtually irradiated
by 10 and 18 MeV 4He2+ ions with the dose of 5x109 cm-2 using the
calibrated system for determination of defect distribution [1].
The parameters of deep levels created by penetrating ions were determined
by means of the deep level transient spectroscopy (DLTS) and are
summarized in Table 1 using the notation of ATLAS. Helium irradiation
produces pure damage defects comprising five deep levels within
the bandgap which are connected with different charge states of
divacancy (E2, E3, H1), acceptor level of vacancy-oxygen VO pair
(E1), and donor level of the carbon-vacancy-oxygen CVO complex (H2).
The data received from measurements for level positions Et
(E.LEVEL) and electron capture cross-sections sign were completed
by capture cross-sections for holes sigp presented in reference
[4] for the same type of defects.

The influence of individual deep levels on electron
lifetime is shown in Figure 1 for both the defect peak (x~180µm)
and defect tail (50µm<x<150µm) regions of the 18
MeV irradiation (see Figure 2) using the general lifetime dependence
on excess carrier concentration that reads

The thermal component of Rn
was calculated from the first equation above for deep level parameters
given in Table 1. Figure 1 enables one to compare the lifetime reduction
in the defect peak with both the unirradiated region and tail part.
Furthermore, t(Dn) as a result of individual and all deep levels
implies that only two levels are dominant.

The level E1 has the biggest impact on the lifetime decrease with
increasing excess carrier concentration above 1014 cm-3 and determines
the so-called high-level lifetime. The level E3 is counteracting,
so it dominates in decreasing the lifetime below 1014 cm-3 . This
is usually referred as a low-level lifetime. For the device under
consideration, the E1 level is responsible not only for the magnitude
of the DC forward voltage drop (n > 1015 cm-3), but also for
the excess carrier recombination within the neutral n-base during
the initial part of the turn-off. E3 brings mainly the desirable
decrease of charge at the far end of reverse recovery (n > 1015
cm-3). Finally, the figure implies the fact that simulation with
the two dominant levels (E1 and E3) gives the same results as with
five ones (verified in simulations of reverse recovery). Since the
influence of both the double-acceptor (E2) and single donor (H1)
levels of divacancy is marginal, the defect can be approximated
as a single acceptor E3. Therefore, a problem with inclusion of
multiple-charged centers, which are not covered by the current ATLAS
SRH model, is avoided.

Device Simulation

Figure 2 shows the excess carrier distribution
n + p of both the unirradiated and helium irradiated devices during
the ON-state (100A@300K). Figure 3 shows the reverse recovery current
and voltage waveforms to be simulated for dc reverse voltage -1000V
and dI/dt=-1000A/ms starting from the conditions of Figure 2. The
overall behavior of irradiated devices is influenced by position
of the defect peak (ion energies) that was intentionally located
in two places with different impact on device parameters. The forward
voltage drop VF is 0.94, 0.98, and 1.032V @100A for unirradiated
devices, 10MeV and 18MeV (dose: 5x109cm-2) irradiations, respectively.
The gradual increase of VF with defect peak distance from the anode
is in agreement with experiment [5]. The influence of ion irradiation
on dynamic behavior is more pronounced. While the unirradiated device
shows oscillators, the 18MeV one is even worse. On the other hand,
using 10MeV the defect peak placed within the n-base close to the
anode softens the diode recovery in agreement with experiment [5].
As a result the removal of the oscillatory behavior takes place.

Figure 3. Current and voltage
waveforms of the reverse recovery process (VRM= -1000V, dI/dt= -1000A/ms)
for unirradiated and He irradiated diodes (10 and 18MeV@5x109cm-2)

Conclusions

It was shown that ATLAS is capable of accurate
simulation of ion irradiated power devices. The user should provide
the electrical parameters of relevant deep levels and define trap
models accordingly in the ATLAS syntax.